Electrolysis



y 0, 1969 P. J. H. CARNELL ET AL ELECTROLYSI S Sheet rs Filed Jan. 4. 1965 Attorneys y 1969 P. J. H. CARNELL ET AL 3,445,354

ELECTROLYSIS Filed Jan. 4, 1965 Sheet 2 of s oentoro tlorneys y 1969 P. J. H- CARNELL ET AL 3,445,354

ELECTROLYSIS Filed Jan. 4, 1965 Sheet 5 of 3 United States Patent 3,445,354 ELECTROLYSIS Peter John Herbert Carnell, Wilfrid Jesse Skinner, and

Michael Staines Spencer, Norton-on-Tees, England, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed Jan. 4, 1965, Ser. No. 423,155 Claims priority, application Great Britain, Jan. 8, 1964, 878/64; Aug. 7, 1964, 32,225/64 Int. Cl. B01k 3/02 US. Cl. 20473 3 Claims ABSTRACT OF THE DISCLOSURE There is provided a process for electrolyzing a dissolved substance to give a dissolved product wherein the susceptibility of the process to diffusion limitation is decreased by subjecting at least one electrode to a wiping action while immersed in the electrolyte whereby the stagnant boundary layer adjacent the electrode is at least partially removed.

This invention relates to electrolytic processes which are subject to diffusion limitation at the surface of solid electrodes.

According to the invention there is provided an electrolytic process of reduced susceptibility to diffusion limitation wherein at least one electrode is subject to a wiping action whilst immersed in the electrolyte.

According to the invention there is also provided an apparatus which may jbe used as an electrolytic cell having a reduced susceptibility to diffusion limitation which contains one or more anodes and cathodes at least one of which is equipped with means for subjecting it to a wiping action.

An electrolytic process is said to be subject to diffusion limitation when there is for example an inadequate rate of diffusion of a-product of the electrolysis away from an electrode surface or of a reactant from the electrolyte to that electrode' fsurface or both. The result can be a decrease in the current efficiency at that electrode surface,

or a decrease in yield or both. These can be compensated by a decrease in current density, but at the cost of decreased output.

It is preferable that the wiping of the electrode should be over substantially the whole of the electrolytically active surface of the electrode. It is preferable that the pressure exerted by the wiper on the electrode surface shall be sufficient to remove substantially all the stagnant boundary layer.

In a preferred form of the invention the wiper is of a deformable material which wipes the surface of the solid electrode without causing appreciable abrasion, for example soft rubber material, brushes or plastic mesh fabric.

It is a matter of convenience whether the solid electrode is fixed and its surface wiped by the moving blade or vice versa. The relative movement is conveniently rotational but may be for example linear. The relative speed required between the electrode and the wiper depends on the current density and increases quite rapidly with increased current density. With practical current densities and wiping efliciency it appears to be sutficient to wipe the surface at intervals of 0.1-3 seconds.

The invention is useful for example in processes where the reaction species is not electrostatically attracted to the electrodes e.g.:

(a) Where the reacting species is non ionic e.g. electrolytic reduction of nitro benzene to azobenzene or electrolytic reductive dimerisation of acrylonitrile to adiponitrile and hexamethylene diamine; or

(b) Where the charge of the reacting species is the same as that of the electrode at whose surface it reacts, e.g. electrolysis of nitrite to produce hyponitrite, of nitrate to produce hydroxylamine or of bisulphite to produce dithionite. Other cases are:

(c) Where the ion has a low diffusion co-efiicient, for example in Kolbes electrolysis of the alkali salts of carboxylic acids to form hydrocarbons etc; and

(d) In general where reaction conditions demand a high current density.

The requirements for cathode materials in'electrolytic process are first that they are not seriously attacked by the catholyte during the process and second" that at a high current density the required reactionlis' the major electrode process. The second requirement can be met by the use of metals with high hydrogen 'pve'rpotentials.

Overpotential may be defined as the difference, under given conditions, between the potential at which a reaction is observed to proceed and the thermodynamically reversible value. Therefore at the surfaces of metals with high hydrogen overpotentials e.g. mercury, lead,- tin, bismuth, zinc, thorium etc. cathode reactions othergthan hydrogen liberation take place at cathode potentialshigher than those thermodynamically required for hydrogen liberation. However we have also found that some metals which, according to published data, hav low hydrogen overpotentials and would therefore be expected to liberate hydrogen at all but the lowest cathodepotentials, for example transition metals, when used as the cathode in the process of the invention, do in fact give good yields of products formed at the cathode at high current densities and cathode potentials.

The mechanism by which such metals are usable is not certain but it appears to be due to either or both of (a) removal of the diffusion limited boundary layer by the wiping action thus removing the reactio'nproduct and allowing the reactant species access to the cathode surface and (b) poisoning of the cathode surface giving it an effective hydrogen Overpotential greater than the published value.

Thus for example, in the process for the production of dithionite by the electrolytic reduction of bisulphite, titanium, chromium and nickel are suitable cathode materials even if the current density is high. Stainless steels from the American Iron and Steel Institute (A181) 300 and 400 series are also suitable. It is believed that some poisoning of the cathode surface by sulphur takes place.

Also for example in the process for the production of hydroxylamine by the electrolytic reduction of nitrate, titanium is a suitable cathode material. The presence of a poison for example mercuric ions in the cathode compartment is advantageous in this process.

The embodiment of the invention using a wiped cathode is particularly useful for the production of dithionites by the electrolytic reduction of bisulphite with process conditions in general the same as when using a mercury cathode as described in our co-pending British application No. 27,203/ 62. It can be used for the production of dithionites of potassium, sodium, ammonium and zinc and particularly of sodium.

In the process for the production of sodium dithionite the catholyte may conveniently contain a small amount for example up to 20% of a water miscible alcohol, for example methyl or ethyl. The catholyte may also contain a small amount for example 0.02% to 5% of an organic base, for example aliphatic or aromatic, primary, secondary or tertiary amines, heterocyclic nitrogen bases e.g. pyridine, and ammonia/ aldehyde or ketone condensation compounds.

The drawings show two electrolytic cells embodying the principles of the invention. The cell shown in FIGURES l and 2 has a stationary cathode and a rotating wiper and the cell shown in FIGURES 3, 4 and has a rotating cathode and stationary wipers.

FIGURE 1 is a plan view of the cell.

FIGURE 2 is a sectional view along AA.

FIGURE 3 is a plan view of the cell.

FIGURE 4 is a sectional view along B--B.

FIGURE 5 is a plan view of the wiper assembly.

The names Perspex" and Permaplex used in the following description are registered trademarks.

The cell shown in FIGURES l and 2 consists of a The top diaphragm ring 100 also carries the wiper supports 108 which are cemented to it and also slotted in the bottom ring 102. The wiper blades 110 are made f foamed latex rubber (Rubazote) and attached to the supsquat Perspex" cylinder 20 which has a porous pot 22 ports 108by means of nylon screws 112 and Perspex" mounted inside with its base a few cms. above the base of strips 114. The wiper supports 108 are prevented from the cylinder 20. To the base of the cylinder 20 is cemented moving outwards by means of Perspc x thrust block 116 a bismuth disc 24, having approximately the same diamscrewed to the cylinder 60. eter as the porous pot 22, to act as the cathoode of the Exam cell. The electrolytically active area (43 cm?) of the bisp muth disc is defined by a strip of adhesive thermoplastic Using an electrolytic cell as shown in FIGURES 1 and sheet 50 around the circumfeernce of the bismuth disc. 2 a d hereinflbove described, the amide COmPQTtmem was A Perspex stirrer 26 is suspended vertically through the charged with ca. 50 mls. of saturated brine, and the ca- I centre of the porous pot 22 and coaxial within a glass thode compartment with 120 mls. of a'solution containtube 28 sealed in the bottom of the porous pot. The two ing 248 gms. Neal-I80 31.5 gms. Na- SO and 200 gms. soft sponge rubber wiper blades 30 are adhesively attached NaCl per litre. The wiping speed was 250 r.p.m. to the stirrer 26 which is arranged so that the wiper blades The cell operated continuously. The metered flow 30 are kept in contact with the surface of the bismuth cathrough the cathode compartment was 8 ml./min. The thode 24 to provide the wiping action. The pressure be- 25 total volume of catholyte in the circuit was 150 mls. The tween the wiper blades and the bismuth cathode 24 is cathode compartment of the cell was blanketed with argon. just sufiicient to cause partial deformation of the wiper The temperature of the cell was maintained at 20 C. blades 30. A measured electric current was passed through the cell The porous pot 22 is the anode compartment and conand the current eihciency measured by analysing the cell tains three graphite blocks 32 as anodes. The space be- 30 efiiuent. Current efficiencies 100%, 89% and 91.5% were tween the porous pot 22 and the cylinder 20 is the cathode obtained with cathode current densities of 0.06, 0.08 and compartment. 0.12 amp. cm? respectively.

The cell is sealed by two rubber bungs 54 the outer of the hfperimeht was repehfed under the same which also acts as a Support f the porous POL dition and using a current density of 0.12 amp. cmr Holes bored in the rubber bungs 54 accommodated an 33 but with the wiper blade Tamed (a) U ahd (b) inert gas inlet 44, a thermometer 42, a catholyte inlet 46, 0-625 h Surface 0f the cathode the a catholyte outlet 38, the catholyte 36, being forced out current efhclehcif fen i (a) 367% and of the cell by the gas lift action of the blanketing inert The current Welds the above P FP havehefih gas, a chlorine outlet to enable chlorine released at the Cmrecied ahow for the known decomposllion of dllhlo' anode 32 to be removed f the anode compartment 4 nite at 2.0 C. At lower temperatures e.g. 5 C. the decomd a li tube 43 to enable Samples f the Cathotyte position of dithionite is very small and hence these current to b removed f 1 i etficiencies would be realised in practice.

The .cell shown in FIGURES 3, 4 and 5 consists of a Example 2 squat Perspex" cylinder cemented to a flat square I base Plate The base Plate 2 has a 018 in This was carried out under the same conditions and the middle with a hollow Perspex funnel 64 cemented using the same apparatus as Example 1 but with the caaround the hole underneath the base D The d of thode compartment charged with 150 mls. of a solution the cell is made from two concentric annular Perspex containing 10 gm. acrylonitrile and 17 gm. NaCl per 150 rings 66 and 68. The ring 66 covers the anode compartmls, and using a current density of 0.04 amp. emf. mcnt 70 and the ring 68 covers the cathode compartment 50 Afte on hour the catholyte was analysed by vapour The rings 65 and 68 are Secured t0 h other and 10 phase chromatography which showed hexamethylene dithe cylinder 60 by means of nylon screws 74 using silicone amine n; b n rubber rings 76 as gaskets.

The four carbon anodes 78, prepared by quartering a Example 3 hollow graphite Cylinder, am Suspended from anode This was carried out under the same conditions and in 11d 6635511011"!- the same apparatus as Example 1 but with a titanium The cathoode 80 is a metal cylinder screwed onto a cathode, the anode compartment was charged with brine stainless steel shaft 82 driven by a motor (not shown). and the cathode compartment charged-with a solution A Perspcx cone 84 is cemented to the base of the cacontaining 248 gms. NaI-ISO 31.5 gms. Na SO and 200 thode 80 to take up dead space and increase the cur- 60 gms. NaCl per litre. rent concentration. A mercury seal 86 is fitted to the steel The wiper was switched on. The cell operated continushaft 82 where it passes through the cathode lid 68. ously the catholyte flow being metered. The cathode com- Four Perspex tubes let into the anode lid constitute partment was blanketed with argon. chlorine outlets 88 and air inlets 90. Three further Per- A measured electric current was passed through the cell spex tubes let into the cathode lid constiute an argon in- 60 and the current efliciency measured by analysing the cell. let 92, an argon outlet 94 and a catholyte inlet 96. efliuent.

RESULTS Current Cell Wiper Dithlontte Flow catholyte Cathode Density Temp, Speed, Conch rat-c.1111. Vol., Efiio. amp. cm r.p.m. gm. lit mink ml. percent 0.05 21 250 10 8.5 150 84.6 0. 1 20. 5 250 20. 3 s. 5 140 85.3 0. 05 20. 0 250 10. n s. 0 152 st. 1 0. 1 21 250 12. 6 14. 7 130 as. J 0. 22. 5 250 15. 2 15. 0 as. 1

Example 4 Example 7 This was carried out under the same conditions and in This was carried out under the same conditions and the same apparatus as Example 1 but using a nickel using the same apparatus as Example 6 but with the staincathode. less steel cylinder cathode chromium plated.

RESULTS Current Cell Wiper Dlthlonite Flow Cathode Catholyte density, Temp., Speed, Cone, Rete,ml. Ellie, Volume. amp. cmr C. r.p.m. gm. lit.- min. percent ml,

RESULTS Current Wiping Dithionite Flow Cathode Catholyte 1 Current, Density, Interval, cncn., rate, Ellie, Volume, amps. J amp.cm.-' Temp.,C. secs. gm.lit." litJhr. percent mls.

Run No i Example Example 8 An electrolytic cell as shown in FIGURES 3, 4 and 5 This was carried out under the same conditions and and as hereinabove described was charged with saturated using the same apparatus as Example 5 but with an brine in the anode compartment and with a solution con- 20 18/10 Mo/Ti stainless steel cylinder as the cathode.

RESULTS Current Wiping Dithionite Flow Cathode Catholyte Density, Interval, Concn., Rate, Etfim, Volume, amp. cmr Temp., C. secs. gm. lit: -lit/.hr. Percent mls.

taining 248 gms. NaI-ISO 31.5 gms. Na- SO and 200 Examp 1e 9 gms. NaCl per litre as catholyte. This was carried out under the same conditions and The cathode was a hollow nickel cylinder three inches using the same apparatus as Example 8 but with 10% of in diameter and three inches long. The motor was switched methyl alcohol by volume added to the catholyte.

RESULTS I Current Cell Wiping Dithionite Flow Cathode Catholyte Current, Density, Temp., Interval, Concn., Rate, Ellie Volume, amps. amp. cmr C. secs. gm. litr lit./lu'. Percent mls.

on to cause. the cathode to rotate. The cell operated conin 'f g gg alcohol transport to the anode occurred tinuously, the catholyte flow being metered. g E 10 A measured electric current was passed through the xamp e cell and the current efficiencymeasured by analysis of the This was carried out under the same conditions and cell eifiuent. using the same apparatus as Example 8 but with the wiper RESULTS Current Wiping Dithionite Flow Cathode Catholyte Density, Temp., Interval, e0nc., gin. rate, the, Volume, amp. em.- 0. secs. lit.- l./hr. percent mls.

Example 6 This was carried out under the same conditions and blades replaced by a series of nylon bristles about 1 cm.

using the same apparatus as Example 5 but with a solid long clamped in a steel frame to give a thin strip of cylinder of 18/8/Ti stainless steel (3 in. diam., 3 in. high) bristles about 3 ins. long.

as the cathode.

RESULTS Current Wiping Dithionite Flow Cathode Cntholyte Current, Density, interval, C0ncn., Rate, Ellie, Volume, amps. amp. em." Temp., C. secs. gm. llt.- lit./hr. percent mls.

Run No 31a 3. 0. 029 19 0. 23 20. 6 0. 55 400 31b 5. 6 0. 043 18. 5 0. 21 4 4. 5 99. 8 400 15 0.115 20 0.25 19.9 2.24 94.0 400 3 14 l 0. 15 27 0. 15 18. 4 2. 07 92. 6 400 40 20.7 0.21 27 0.22 23.7 2.4 91.6 400 I High current density obtained by only using three anodes.

' RESULTS Current Cell. Wiping Dlthionlte Flow Cathode Catholyte Density, Temp... Interval, Concn., Rate, Efiie, Volume amp. cm 4 C. secs. gm. lit. lit./hr. Percent mls.

N iifia? 0. 03 21 0. 42 2. 9 4. 2 98. 6 400 65b 0. 10 25 0. 42 18. 3 2. 14 99. 4 400 0. 21 31 0. 42 21. 6 3. 96 98. 4 400 The current yields in Examples 3 to 11 have been Example 11 p This was carried out under the same conditions and using the same apparatus as Example 8 but with the wiper assembly removed and the gap between the cathode and the membrane filled with polypropylene mesh.

corrected to allow for the known decomposition of dithionite at the cell temperature. At lower temperatures e.g. 5 C. the decomposition of dithionite is very small and hence these current efficiencies would be realised in practice.

RESULTS Current Cell Speed of Dlthionite Flow Cathode Catholyto Density, Temp., rotation ol Concn.. Rate, Effie, Volume, amp. cum- C. Cathode, gm. lit. lit./hr. Percent mls.

r.p.m.

Run No.

648 0.09 26 100 20. 0 1. 7s 99. 5 400 64h 0.11 21 100 21. 6 1. 90 9a. 9 400 Example 12 Using the same apparatus as in Example 3, the anode 2.1 compartment was charged with saturated sodium carbonate solution and the cathode compartment with an aqueous We claim:

solution of sodium nitrate (34% w./w.) and a trace amount of mercuric chloride.

The wiper was switched on and the cathode compartment blanketed with argon. The cell operated continuously, the catholyte flow being metered.

A measured electric current was passed through the cell and the current efficiency measured by analysis of the cell eflluent.

The results obtained are given as run No. l in the Table. On switching the wiper otf evolution of hydrogen was observed.

The results shown for run No. 2 were obtained by reall 1. A process for electrolysing a dissolved substance to give a dissolved product wherein the susceptibility of the process to diffusion limitation is decreased by subjecting at least one electrode, which electrode surface is poisoned, to a wiping action while immersed in the electrolyte, whereby the stagnant boundary layer 'adjacent the electrode is at least partially removed.

there is present in the catholyte a surface poisoning agent. i

3. An electrolytic process according to claim 1 wherein the poisoning agent is mercuric ions.

peating run No. 1 but with no mercuric ions in the cathode compartment. Some hydrogen was evolved, the rate of evolution being independent of the wiper speed.

RESULTS Hydroxyl- Current Wiper amine Flow Cathode Catholyte Current, Density. Temp Speed. concn.. gm. Bate. Efiie. Volume.

amps amp cm: r.p.m. lit. l./hr. percent mls.

Run 50 References Cited UNITED STATES PATENTS 2,399,289 4/1946 Negus 204-275 323,514 8/1885 Majcrt 204-72 860,657 7/1907 Hatfield 204-291 1,668,434 5/1928 Todd 204--l95 2,477,579 8/1949 Condit 20473 2,606,l48 8/ i952 Portanova et al 204-2l2 3,193,481 7/1965 Baizer 204-73 3,313,715 4/1967 Schwartz 204-36 OTHER REFERENCES Bomberger, H.B.: Titanium, Industrial and Engineering Chemistry, vol. 49, #9, Pt. 11., September 1957, pp.

JOHN H. MACK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner.

US. Cl. X.R.

2. An electrolytic process according to claim 1 wherein: 

